There are many situations in which it is desirable to have uniform light exitance from an illuminating device. That is, it is often desirable that the luminance of light which escapes from an illuminating device be substantially constant at all light escapement points on the device. Consider for example rear illuminated panel signs of the type commonly used for advertising purposes. Light from one or more light sources located behind a translucent, message-bearing panel escapes through the panel. Ideally, the luminance of the escaping light is substantially identical at all points on the outer surface of the panel so that the viewer perceives a pleasing, uniformly illuminated image, devoid of regions which appear to be darker or brighter than other regions on the panel.
It is also desirable to minimize the number of light sources employed in illuminating devices like those described above, in order to reduce the cost of manufacturing and/or servicing the device, to reduce the size of the device, and to increase the reliability of the device (a single, expensive light source is typically more reliable than a multiplicity of inexpensive sources and can be less expensive as well). However, if a single light source, or a small number of concentrated light sources are used, then the light must be distributed in order to achieve uniform light exitance from the device. Commonly, reflectors are positioned within the device so that light emanating from the light source is, in effect, reflected many times within the device before escaping through a light escapement port such as the translucent advertising sign panel mentioned above. This ensures that the quantity of light escaping at different points on the device is substantially the same, resulting in uniform illumination.
Two classes of prior art device employ the foregoing technique. The first class of device, of which conventional lampshades and back-lit panels are typical, uses reflective material which is diffuse--such that light is reflected from the material in a random direction (i.e. the reflected light has an approximately Lambertian angular intensity distribution; where the intensity is proportional to the cosine of the angle from the normal direction to the surface through which the light escapes). However, this approach is very limited in terms of the distance over which light can be distributed. The problem, as explained below, is that in order to distribute the light an average of "n" diameters, the device must reflect the light an average of n-squared times. Since a fraction of the incident light is inevitably absorbed by the reflector each time it reflects the light, the efficiency of this class of device drops extremely rapidly as the light distribution distance increases.
The second class of device avoids the foregoing problem by employing longitudinally specular reflective materials. Such material reflects light with the important characteristic that each reflected light ray has the same component of motion in a preferred direction relative to the material. Typically, this direction corresponds to a well defined longitudinal axis of the device. Since the component of motion in the direction of the longitudinal axis of the device is now constant for each reflected light ray (rather than randomly changing, as in the first class of device) the distribution distance for a particular light ray increases in proportion to the number of reflections of that ray. Accordingly, light can be distributed over greater distances, with reasonable efficiency. Unfortunately however, longitudinally specular light reflecting materials capable of reflecting light in the foregoing manner over a broad range of angles of incidence are extremely expensive. Although there are some inexpensive longitudinally specular materials capable of reflecting light in the foregoing manner over a narrow range of angles of incidence, these greatly restrict the overall geometry of the resultant illuminating device and also restrict the types of light sources which can be employed.